Christopher B. Leinberger is a man on a mission - a real estate developer who is building to reclaim the past. When not spearheading an ambitious redevelopment of downtown Albuquerque, N.M., he crisscrosses the country, trying to sell builders, planners and the public on converting the nation's sprawling, car-addicted suburbs into more compact, walkable communities - like the neighborhood he grew up in outside Philadelphia. At stake, contends the silver-haired, Santa Fe, N.M.-based developer, is nothing less than our personal health, and that of the planet. "The way we're building our suburbs presents such difficult problems," Leinberger said yesterday, as...